Dual-Phase, Compositionally Complex Alloys: An Exploration of L21 Heusler-Phase Reinforcement

Many historical alloy development efforts have focused on the improvement of the mechanical properties of alloys, with fewer investigations focused on the improvement of corrosion performance, and fewer still that are focused on a concurrent design focused on both mechanical and corrosion properties. The present study is part of a larger effort which seeks to produce lightweight, low-cost alloys possessing room temperature strength, ductility, toughness and aqueous corrosion resistance. Modern alloy design efforts have produced a new class of metal alloys which are composed of many elements in significant proportion.
In the present case, it is advantageous in an alloy for marine applications to not experience a ductile-to-brittle transition (DBT) at or near its operation temperature, which could be as low as -30 °C. This consideration led to a focus on alloys with a face-centered cubic (FCC) crystal structure, which generally speaking do not suffer from a DBT. However, it was also recognized that many single-phase FCC alloys have only modest mechanical strength. Therefore, within an elemental palette informed by the aqueous corrosion performance of traditional alloys, it was sought to identify candidate 2-phase mixtures which could simultaneously realize strength, ductility and aqueous corrosion resistance. The alloy design process was guided by an innovative machine learning approach. As alloy compositions were varied, a ferritic phase began to emerge within one of the alloys predicted to be single phase FCC, and this second phase was stable even after homogenization annealing at 1070 °C for multiple hours. The alloy, Al6Cr10Fe40Mn5Mo3Ni30Ti6, showed good corrosion performance, on par with stainless steel 316L.
The present dissertation research seizes upon this opportunity by exploring the adjacent chemical and phase space to determine if this corrosion-resistant phase can be leveraged for mechanical strengthening. Additionally, it was sought to determine how amenable these two-phase mixtures were to conventional thermomechanical processing strategies of cold rolling and static annealing, and to what extent their microstructures could be controlled. In-situ neutron diffraction experiments were performed to investigate stress and strain partitioning between phases in FCC matrix plus intermetallic compound reinforced composite alloys.
Scanning- and transmission electron microscopy, as well as x-ray diffraction experiments were able to identify the second phase as having the L21 “Heusler” phase (Cu2MnAl archetype) structure. Although Heusler phase compounds are known to be quite brittle at room temperature, there has been little exploration of pairing the Heusler phase with a disordered FCC matrix. In the alloys synthesized, the room temperature brittleness of the Heusler phase was suppressed for two reasons: first it was determined that a level of hydrostatic pressure [1][2] is imposed upon the particles by the deforming ductile austenite matrix and this suppresses the tendency to fracture. The second reason will be detailed below.
The chemical phase space about the original composition was explored using Thermo-Calc software, and Al was identified as the most potent single element for promotion of the Ni2TiAl Heusler phase. Alloys were synthesized across the predicted two-phase region to establish control over the volume fraction of reinforcing phase present. A strategy based in the “lever rule” was also devised, whereby the volume fraction of phases may be controlled without altering phase chemistry.
In addition to the amount of reinforcement, it was also sought to establish control over the microstructure of the FCC + L21 alloys. The dual-phase alloys were found to be generally cold deformable up to 70% reduction in thickness without issue, though alloys with larger Heusler phase did prove to be less formable. The Heusler phase was found to promote finer microstructures after recrystallization annealing, and the alloys seem to adopt an unusual mechanism of continuous recrystallization. The texture of the deformed alloys, as well as the deformed and recrystallized alloys, was found to be generally weak. The FCC matrix showed textures similar to austenitic stainless steels, especially those which contain Mo. Plastic deformation in the L21 phase was confirmed, and the Heusler phase textures were reminiscent of a BCC metal which had undergone {hkl}<111> “pencil glide”. Despite the fact that the Heusler phase is an FCC lattice with an ordered basis, it can be visualized as a collection of BCC-like unit cells. Occurrence of BCC type slip behavior suggests that the antiphase domain boundary (APB) energy is low. In this way, the L21 Heusler phase in these alloys behaves similarly to B2-structured CuZn and FeAl. This ability to accommodate some plasticity via dislocation glide is the second reason why the present L21 material is not so brittle as previous observations may have suggested.
In-situ neutron diffraction experiments were performed both on a model, FCC + B2 steel (based upon the Fe-Mn-Al-Ni-C system) and the novel FCC + L21 alloys. Examination of the lattice strains of the FCC+B2 alloys revealed elastically anisotropic behavior in the FCC matrix phase, with considerably less anisotropy in the B2 reinforcement. Elastic constants were derived from the measured diffraction elastic moduli: For the FCC matrix, C11 = 174 GPa, C12 = 112 GPa, C44 = 99 GPa, µ = 56 GPa, E = 159 GPa and Zener anisotropy ratio Z = 3.17, and for the B2 phase, C11 = 260 GPa, C12 = 162 GPa, C44 = 77 GPa, µ = 61 GPa, E = 174 GPa and Z = 1.57. The tests also reveal that the high carbon added to the Fe-Mn-Al-Ni-C alloy is the major source of strength. Elastic constants were also derived for the FCC matrix of the FCC+L21 CCAs: C11 = 178 GPa, C12 = 93 GPa, C44 = 88 GPa, µ = 61 GPa, E = 167 GPa and Z = 2.08. Both classes of alloy exhibit a Bauschinger effect, which is typical of such composite type alloys, due to elastoplastic mismatch between matrix and strengthening phases. The in-situ neutron diffraction data permits parsing of the contributions of composite (interphase) and intragranular (dislocation-based) sources of back-stress.

Sponsored by the Office of Naval Research - ONR BAA #N00014-18-1-2621, directed by Dr. Airan Perez and Dr. David Shifler.